The present invention is directed to the preparation of organic isocyanates. In particular, the invention concerns preparation of diphenylmethane diisocyanates and related higher homologs, polymethylene polyphenyl isocyanates, from corresponding carbamates in a multistep thermal decomposition process.
Diphenylmethane diisocyanates and polymethylene polyphenyl isocyanates, generally referred to as "MDI" and "PMPPI", are widely used in the production of polyurethanes.
Industrial processes for manufacture of MDI and PMPPI are conventionally based on reaction of phosgene with methylenedianiline or salts thereof. Due to the volatility and toxicity of phosgene, other methods have recently been developed, which do not require the use of phosgene.
The known non-phosgene processes involve thermal decomposition of carbamates.
Thermal decomposition of carbamates is either carried out in the vapour phase at high temperatures or in the liquid phase at lower temperatures.
Due to the high temperatures necessary in the vapour phase decomposition processes and serious side reactions caused by the high temperature conditions, the liquid phase process has gained increasing interest in the commercial production of isocyanates.
In the liquid phase processes described in the literature, the carbamates are decomposed to the corresponding isocyanate and alcohol typically at temperatures between 170.degree. C. and 350.degree. C., while being dissolved in an inert solvent (GB Patent No. 1,458,595; U.S. Pat. No. 4,292,254).
To obtain the desired product yield, the reaction time is generally between 30 minutes and a few hours. Produced isocyanates and alcohol formed during the thermal decomposition of carbamates are then recovered separately by distillation of the solution and/or stripping with an inert stripping agent.
The decomposition of carbamates is a fairly slow process and catalysts have been proposed to speed up the reaction (EP Patent No. 28,337). Additionally, the equilibrium at conditions employed during thermal decomposition of carbamates is unfavourable for extensive decomposition to the alcohol plus isocyanates in the solvent.
Consequently, to permit high yield of the desired isocyanates the alcohol-to-isocyanate ratio in the liquid phase should be constantly at a minimum.